Cooking item comprising a non stick coating with improved properties of adhesion to the substrate

ABSTRACT

A cooking item including a vitreous coating with improved impact-resistance properties, and a method for manufacturing such an item. The method for manufacturing includes optionally treating a concave internal side of a substrate having the end shape of the cooking item, the concave internal side being arranged on the side of the food to be introduced in said item, and a convex exterior side to be arranged on the heat source side, so as to obtain a treated internal side adapted to cause a hard base layer to adhere to the substrate, The method further includes preparing an adhering hard base layer on said internal side of the substrate, be it treated beforehand or not, and preparing a non-stick coating on the hard base layer.

RELATED APPLICATIONS

This application is a division of application Ser. No. 13/000,888 filedApr. 4, 2011, which in turn is a National Phase entry of PCT ApplicationNo. PCT/FR2009/051981, filed Oct. 16, 2009, which claims priority fromFrench Application No. 0857044, filed Oct. 16, 2008, each of which ishereby fully incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to cooking items comprising anon stick coating, and more particularly, it relates to cooking itemscomprising a vitreous, non stick coating having improved properties ofadhesion to the substrate. The present invention also relates to amanufacturing method for the same.

BACKGROUND ART

As used herein, the meaning of vitreous coating is a coating having aglass or enamel appearance which may be either organo-mineral, ortotally mineral.

As used herein, the meaning of organo-mineral vitreous is a coatingcomposed of a sol-gel type material (that is, obtained through a sol-geltreatment), whose network is substantially inorganic, yet including anorganic groups, particularly because of the curing temperature of thecoating and the precursors used.

As used herein, the meaning of entirely mineral coating is a coatingcomposed of an entirely inorganic material, free from all organicgroups. Such a coating may also be obtained through a sol-gel processwith a curing temperature of at least 400° C., or from tetraethoxisilane(TEOS) precursors with a curing temperature which may be less than 400°C.

In the field of vitreous, non stick coatings, intended for use inculinary articles, sol-gel coatings, particularly those obtained fromsilica (silane) based, or alumina based (aluminate) metal alkoxides.

These coatings are currently experiencing wide development in the fieldof cooking items, in that they make it possible to obtain coloredcoatings with a particularly good resistance to temperature andscratches.

However, such coatings have limited adhesion on metal substrates, inparticular, aluminum substrates, stainless steel substrates or cast ironsubstrates.

To alleviate these drawbacks, it has been known by the skilled person toprepare the substrate surface through chemical treatment (for example, achemical etching treatment) or mechanical treatment (for example abrushing or sand blasting treatment), or even a combination thereof.

Nevertheless, even when such treatments are carried out, the adhesion ofthe non stick coating to the substrate remains limited, particularlywhen the non stick coating is subjected to a mechanical deformation sucha an impact or drilling, for example, in order to fix a rivet or anassembling stud.

Thus, when cooking items, whose bottom has an internal side coated witha sol-gel type, non stick coating, are subjected violent shocks, a markis observed under the impact, accompanied with cracks propagatingradially even though the internal side of the bottom has been brushed orsand blasted beforehand.

Thus, the manufacturing of such cooking items should be carried out verycautiously causing high scrap rates and low manufacturing throughputs.

To alleviate the problems of limited adhesion to the substrateexperienced with sol-gel type, vitreous coatings, the applicant hassurprisingly discovered that reinforcing the internal side of thecooking item bottom with a discontinuous, hard enameled base makes itpossible to significantly improve the adhesion of the non stick coatingon metallic substrates (in particular, aluminum or aluminum alloysubstrates), notably exhibiting an enhanced impact resistance of the nonstick coating.

As used herein, what is meant by impact resistance is the ability of thecoating to resist a violent shock.

By contrast, after a violent shock or impact) a coating having a lowimpact resistance exhibits, under the impact, a mark accompanied byradially propagating cracks. The adhesion in this area becomes so weakthat the mere fact of scraping with a nail removes a portion of thecoating whereof the size is actually higher than that of the impactitself.

More particularly, the object of the present invention is a cooking itemcomprising a metallic substrate having a concave internal side to bearranged on the side of the food, and a convex exterior side to bearranged on the heat source side, said internal side being successivelycoated from the substrate with a hard base layer and a non stick coatingcovering said hard base layer, wherein:

-   -   the hard base layer is a discontinuous enamel layer in the form        of a superficial dispersion of enamel drops homogeneously        distributed over the internal side of said item, the internal        side being covered with a coverage ratio between 40% and 80%, a        surface density between 300 drops/mm² and 2000 drops/mm² and a        drop size between 2 μm and 50 μm, and    -   said anti stick coating is a PTFE-free, vitreous type coating        shaped as a continuous film having a thickness of at least 10 μm        and composed of a sol-gel material comprising a matrix of at        least a metal polyalkoxilate and at least 5% by weight of the        coating total weight of at least one metal oxide dispersed in        said matrix.

As used herein, what is meant by enamel is enamel having a softeningpoint lower than the melting temperature of the substrate.

For instance, in the case of an aluminum or aluminum alloy substratewith a melting temperature of about 600° C., the softening point ofenamel is lower than this temperature.

As used herein, what is meant by superficial dispersion of enamel dropsis a discontinuous enamel layer in a divided state over a substrate (inthis case, the substrate of the cooking item), such that the roughnessof this layer is produced by the dispersed enamel drops.

As used herein, what is meant by coverage ratio of the substrate is theratio, expressed in percent, of the substrate area actually covered bythe superficial dispersion of enamel drops by the total area of thesubstrate which may be covered by the hard base layer.

As used herein, what is meant by surface roughness Ra, is the arithmeticmean deviation between the grooves and peaks of the surface with respectto the median (or mean) line, this deviation being assessed based on ISO4287 standard.

It is observed that the presence of such a discontinuous enameled hardbase layer arranged between the substrate and the sol-gel type non stickcoating yields to a significant improvement of the impact resistance ofthe non stick coating.

With more particular regard to the enameled and discontinuous hard baselayer, it exhibits a surface roughness Ra which actually depends on thatof the substrate internal side whereon it is deposited.

Thus, in the case of a substrate whereof the internal side has been sandblasted prior to the deposition of the hard base layer and whichexhibits a surface roughness Ra between 3 and 8 μm, the enameled anddiscontinuous hard base layer has then a surface roughness Ra comprisedbetween 9 and 12 μm.

On the contrary, in the case of a substrate whereof internal side issmooth, the enameled and discontinuous hard base layer by which it iscovered thus exhibits a surface roughness Ra comprised between 2 and 4μm.

Advantageously, the enameled and discontinuous hard base layer has ahardness higher than or equal to that of the metal and/or metal alloycomposing the substrate.

Now relating to the coating covering the structured internal side, andparticularly, the sol-gel material composing this coating, this materialmatrix may advantageously comprise the condensation products of metalpolyalkoxilates, for example, one or more polyalkoxisilanes, analuminate, a titanate, a zirconate, a vandate, a borate, or mixturesthereof.

Preferably, the coating matrix according to the invention comprises apolyalkoxisilane and/or an aluminate such as to compose a mixed matrix.

In an alternative embodiment of the invention, the coating matrixaccording to the invention is grafted through one or several organicgroups selected from C₁-C₄ alkyl groups and phenyl groups. These groupsare required for improving the hydrophobic capability of the coating. Inview to obtaining a better thermal stability for the coating, shorterchains are preferred in the frame of the present invention.

Preferably, the coating matrix according to the invention is graftedthrough one or more methyl groups, which enhance the hydrophobic featureof the coating without interfering with the formation of the inorganicnetwork.

Besides the matrix of at least one metal polyalcoxylate, the vitreouscoating according to an embodiment of the invention, comprises at least5% by weight, and preferably from 5% to 30% by weight of the totalweight of the coating, of at least one metal oxide, which is preferablyfinely dispersed in the matrix. This metal oxide is of a generallycolloidal shape in the form of aggregates, whose size is lower than themicron, or even from 300 to 400 nm.

As colloidal metal oxide for use in the non stick coating according tothe invention, silica, alumina, cerium oxide, zinc oxide, vanadium oxideand zirconium oxide may be cited. Preferred colloidal metal oxides aresilica and alumina.

The presence of a metal oxide in the coating matrix an embodiment of theinvention makes it possible to obtain a film with sufficient thickness,that is, a thickness of at least 10 μm. if the coating thickness islower than 10 μm, the mechanical strength of the formed film would notbe sufficient.

Preferably, the film has a thickness comprised between 10 and 80 μm, andmore preferably between 30 and 50 μm such that the film layer thusformed is continuous, consistent and sufficient to absorb the substrateroughness.

Advantageously, the sol-gel material composing the non stick coating mayfurther comprise at least silicone oil in order to enhance thehydrophobic nature of the coating surface, and particularly following athermal shock such as flame exposure type.

In this regard, the metal polyalkoxylate has hydrophobic groups whichare destroyed at high temperature during the passage over a flame.Nevertheless, this disappearance of the hydrophobic nature is temporary,as it is compensated for by the silicone oil trapped in thepolyalkoxylate and whereof surface migration in infinitesimal amountspromotes the progressive rebuild of hydrophobic groups on the filmsurface.

With a coating according to an embodiment of the invention comprising atleast 0.1% by weight of silicone oil, it is observed that the rebuild ofthe hydrophobic nature is sufficient at the time of a new cooking. Inthis regard, the value of the static contact angle θ of a water dropdeposited on the coating of the invention is of about 20° after a flameexposure type thermal shock. This static contact angle value goes up toat least 75° after a process of rebuilding the hydrophobic features ofreheating the room temperature at 200° C., for a period of at least 5minutes, that is, when the item is ready for a new cooking.

Preferably, the silicone oil represents 0.1 to 6% by weight and morepreferably 0.3 to 5% by weight of the total weight of the coating (drystate). Less than 0.1% by weight of silicone oil, the rebuild ofhydrophobic groups that have disappeared during flame passage (600° C.)is lower, the obtained angle being lower than 62° C.

More preferably, the sol-gel material of the coating according to anembodiment of the invention comprises 0.5 to 2% by weight of siliconeoil with respect to the total weight of the dry coating. In this case,the initial static contact angle θ of a water drop deposited on such acoating is of 95° C. Following flame passage type a thermal shock, thiscoating exhibits an angle of 20° C. Following a rebuild processcomprising at least a step of reheating the room temperature to 200° C.over a period of at least 5 minutes, the static contact angle becomeshigher than 75° C. when the item is ready for a new cooking.

The coating according to an embodiment of the invention may comprise asilicone oil or a mixture of silicone oils.

As silicone oils that may be used in the coating according to anembodiment of the invention, may be particularly cited phenyl silicones,methyl-phenyl silicones and methyl silicones.

If the coating according to an embodiment of the invention is to be incontact with food items, food grade silicone oil should preferably beselected, and in particular, one selected from food grade methyl-phenylsilicones and methyl silicones.

As methyl-phenyl silicone oils, may be particularly cited non food oilsfrom WACKER under trade name WACKER SILICONOL AP150 and from DOW CORNINGunder trade name DOW CORNING 550 fluid, as well as food oils from WACKERARM. As methyl silicone oils, oil from RHODIA under trade name RHODIA 47V350, oil WACKER 200 fluid from WACKER or TEGO ZV 9207 oil from TEGO,may be cited, which are food-grade methylsilicone oils.

Preferably, a silicone oil selected from the aforementioned oils isused, which oil has a molecular weight of at least 1000 g/mol, which isnon reactive and exhibits a viscosity between 20 et 2000 mPa·s.

Advantageously, the sol-gel material of the coating according to theinvention may further comprise fillers for enhancing the mechanicalfeatures of the formed coating, and/or pigments, to render the coatingcolored. Moreover, the presence of fillers and/or pigments has also anadvantageous effect on the hardness of the film.

As fillers which may be used in the coating according to an embodimentof the invention, alumina, zirconium, mica, clay (such asmontmorillonite, sepiolite, gypsite, kaolinite and laponite®) andzirconium phosphate may be cited.

As pigments which may be used in the coating according to an embodimentof the invention, titanium dioxide, copper-chromium-manganese mixedoxides, iron oxide, carbon black, pyralene red, aluminosilicates, metalflakes and particularly aluminum flakes, may be cited.

Preferably, the fillers and/or pigments are flake shaped, whichadvantageously improves the hardness of the non stick coating.

Preferably, the fillers and/or pigments are nanoscaled so as to improvetheir dispersion and their distribution in the coating, providing thecoating a high performance consistency.

In an advantageous version of the cooking item according to anembodiment of the invention, the substrate is a hollow cap of a cookingitem, having a bottom and a lateral wall rising from said bottom.

The substrate of the cooking item according to an embodiment of theinvention is advantageously made from a material selected from metals,glass and ceramic.

Metal substrates, and preferably anodized or non anodized aluminum oraluminum alloy, stainless steel, cast iron, iron or copper substratesare recommended.

As aluminum alloys that may be used to make the substrate of the cookingitem according to an embodiment of the invention, are particularlyrecommended low alloyed, enamelable aluminum alloys, and in particular:

-   -   99% aluminum-containing “pure” aluminums of series 1000, and for        example, alloys 1050, 1100, 1200 and 1350,    -   aluminum and manganese alloys of series 3000, and for example,        alloys 3003, 3004, 3105 and 3005,    -   aluminum and silicon alloys of series 4000,    -   aluminum and magnesium alloys of series 5000, and for example        alloys 5005, 5050 and 5052, and    -   aluminum, silicon and magnesium alloys of series 6000, and for        example, alloys 6053, 6060, 6063, 6101 and 6951, and    -   aluminum, iron, silicon alloys of series 8000, and for example,        alloy 8128.

Finally, embodiments of the present invention also relate to a methodfor manufacturing a cooking item, comprising the steps of:

a) a step of providing a substrate having the end shape of the cookingitem with a concave internal side to be disposed on the side of the foodto be introduced in said item, and a convex exterior side to be disposedon the heat source side;

b) optionally, a step of treating the internal side of the substrate, soas to obtain a treated internal side adapted to cause a hard base layerto adhere on the substrate;

c) a step of preparing an adhering hard base layer on the substrateinternal side, be it treated beforehand or not;

d) a step of preparing a non stick coating on the hard base layer formedin step c); the method including:

-   -   step c) of preparing a discontinuous, enameled hard base layer        comprising the following successive steps of:        -   c1) preparing an aqueous slurry of enamel frit;        -   c2) applying, through air spraying, the slurry on the            substrate internal side with a pressure equal to or higher            than 4 bars such that the amount of deposited slurry is            comprised between 0.07 g/dm² and 0.2 g/dm²; then        -   c3) drying and/or curing said enamel layer so as to form a            hard biscuit of enamel or a layer of cured enamel; and    -   step d) of preparing the non stick coating on said enamel hard        biscuit or said cured enamel layer comprising the following        successive steps of:        -   d1) preparing a sol-gel composition (A+B) comprising at            least a colloidal metal oxide and at least a metal oxide            type precursor;        -   d2) applying over the entirety or over a portion of the            enamel hard biscuit or said cured enamel layer at least a            layer of the sol-gel composition (A+B) whose wet-basis            thickness is at least of 20 μm; then        -   d3) curing said sol-gel composition layer (A+B) so as to            obtain a non stick vitreous coating whose thickness is at            least of 10 μm.

Advantageously, the enamel frit has less than 50 ppm of cadmium and lessthan 50 ppm of lead, and comprising 30 to 40% by weight of silica and 15to 30% by weight of titanium oxide, less than 10% by weight of vanadiumoxide and less than 4% by weight of lithium oxide with respect to thetotal weight of the frit, and the aqueous slurry comprises at least 20%by weight of mineral fillers with respect to the total weight of theslurry.

With regard to the preparation of the hard base layer on the substrateinternal side, according to embodiments of the present invention, nohomogeneous compound whose post-curing structure is determined by itspre-curing, initial chemical composition is applied, by contrast to anelectric-arc or plasma projection method. In this regard, in embodimentsof the present invention, an aqueous slurry of enamel frit is appliedand it is observed during drying and/or curing that a homogenization ofthe different fusible elements of the slurry, those arising from theenamel frit and those arising from the slurry formulation thereof, takesplace. Thus, in the present invention, there is no one-for-onecorrespondence between the slurry composition and the structure of theenamel formed following the application and the curing of this slurry.

The method pursuant to embodiments of the present invention has theadvantage of implementing an aqueous slurry of an enamel frit which doesnot contain any solvent and thus generates no VOC, and, in embodiments,the enamel frit used in the method according to the present inventioncontains almost no noxious matter such as lead or cadmium or hardly inthe form of little traces (50 ppm at the most of a noxious element),such that the enamel thus formed conforms to the food legislation interms of enamel frit formulation or in terms of slurry formulation.

According to an alternative embodiment of the method of the invention,during step c) of preparing a discontinuous, enameled hard base layer,the non cured enamel layer is merely dried at a temperature comprisedbetween 120° C. and 200° C. to yield an enameled biscuit.

According to a second alternative embodiment of the method of theinvention, during step c) of preparing a discontinuous, enameled hardbase layer, the non cured enamel layer is subjected to curing at atemperature comprised between 500 and 580° C.

With regard to the preparation of the sol-gel type non stick coating, inembodiments, the sol-gel composition A+B is prepared as follows:

i) preparing an aqueous composition A comprising 5 to 30% by weight ofthe total weight of the aqueous composition A of at least a metal oxide,and 0 to 20% by weight of the total weight of composition A of a solventcomprising at least one alcohol; then

ii) preparing a solution B comprising at least a metal alkoxide typeprecursor; then

iii) mixing solution B of metal alkoxide with aqueous solution A so asto obtain a sol-gel composition (A+B) containing 40 to 75% by weight ofthe aqueous composition A to the total weight of the sol-gel composition(A+B), so that the amount of colloidal metal oxide represents 5 to 30%by weight of the sol-gel composition (A+B) on a dry basis.

With more particular regard to the preparation of the aqueouscomposition A, the introduction of at least 5% by weight of at least ametal oxide with respect to the total weight of composition A isrequired to form a film whose post-curing thickness is at least of 10microns. If, on the contrary, the amount is higher than 30% by weight ofthe total weight of composition A, composition A is not stable.

The metal oxide of the aqueous composition A is as defined above.Preferably, it is a colloidal metal oxide selected from colloidal silicaand/or colloidal alumina.

The presence of an alcohol-based solvent is optional, but has theadvantage of improving the compatibility of aqueous composition A withmetal alkoxide solution B.

Nevertheless, it is also possible not to use a solvent, yet, in thiscase, the number of possible polyalkoxylates is reduced to those havingan excellent compatibility with water. It is possible to have an excessamount of solvent (higher than 20%), yet, this unnecessarily generatesnon environmental-friendly volatile organic compounds.

As a solvent, an oxygenated alcohol-based solvent or ether-alcohol ispreferably used in aqueous composition A.

Aqueous composition A according to embodiments of the invention may alsocomprise, besides the colloidal metal oxide, and, if need be, thealcohol-based solvent, at least a silicone oil, which is preferablypresent in composition A in an amount of 0.05% to 3% by weight withrespect to the total weight of the composition.

With an aqueous composition A comprising 0.5 to 2% by weight of siliconeoil, a coating exhibiting hydrophobic features which may rebuild in theframe of a cooking use. Silicone oil of composition A is a food-gradesilicone oil defined above.

Aqueous composition A in embodiments of the invention may furthercomprise fillers and/or pigments such as defined above.

Aqueous composition A in embodiments of the invention may furthercomprise pyrogenic silica, which is intended to adjust the viscosity ofthe sol-gel composition and/or the glossy effect of the dry coating.

With regard to the preparation of solution B, as a precursor ispreferably used a metal alkoxide selected from the group consisting in:

-   -   precursors whose chemical formula is M₁(OR₁)_(n),    -   precursors whose chemical formula is M₂(OR₂)(OR₂)_((n-1))R₂′,    -   precursors whose chemical formula is M₃(OR₃)(OR₂)_((n-2))R₃′₂,

R₁, R₂, R₃ or R₃′ being an alkyl group,

R₂′ designating an alkyl or phenyl group,

n being an integer corresponding to the maximal valence of metals M₁,M₂, or M₃,

M₁, M₂, or M₃ designating a metal selected among Si, Zr, Ti, Sn, Al, Ce,V, Nb, Hf, Mg or Ln,

Advantageously, the metal alkoxide of solution B is an alkoxysilane.

As alkoxysilanes which may be used in solution B of the method of theinvention, methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS),methyltriethoxysilane (MTES), dimethyldimethoxysilane, and mixturesthereof may be cited.

Preferably, alkoxysilanes MTES and TEOS will be used as they do notcontain methoxy groups. In this regard, the hydrolysis of methoxy leadsto the formation of methanol in the sol-gel formulation, which, due toits rating as a toxic element, requires additional precautions duringapplication. On the contrary, the hydrolysis of ethoxy groups generatesonly ethanol having a more favorable rating and hence, less restrictingusage prescriptions with regard to the sol-gel coating.

According to an advantageous embodiment of the method of the invention,solution B may comprise a mixture of an alkoxylane such as defined aboveas well as an aluminum alkoxide.

The metal alkoxide type precursor of solution B is a mixture of amineral, organic Lewis acid representing 0.01 to 10% by weight of thetotal weight of solution B.

As acids which may be used for mixing with the metal oxide precursor,acetic acid, citric acid, ethyl aceto-acetate, hydrochloric acid orformic acid may particularly be cited.

Preferred acids of embodiments of the invention are organic acids, andmore particularly, acetic acid and formic acid.

After preparing aqueous composition A and that of precursor solution B,they are mixed together, in order to form a sol-gel composition (A+B).The respective amounts of each of compositions A and B should beadjusted such that the amount of colloidal silica in the sol-gelcomposition represents 5 to 30% by dry weight.

The sol-gel composition (A+B) of embodiments of the invention may beapplied over the substrate through spraying or through any otherapplication mode, such as by soaking, stamping, with a paintbrush, witha roller, by spin-coating or serigraphy. Nevertheless, in the frame of ashaped object, for example, gun spraying has the advantage of forming acontinuous, homogenous film which forms, after curing, a continuouscoating whose thickness is smooth and tight.

Generally, after applying the sol-gel composition (A+B) according toembodiments of the invention, drying is carried out preferably at atemperature of 60° C. for a period of 1 minute.

The nature of the non stick coating changes according to the curingtemperature, from an organo-mineral coating, for a curing temperature ofabout 200° C. to a substantially mineral coating for higher curingtemperatures.

At a curing temperature lower than 400° C., particularly between 180 and350° C., the non stick coating is an organo-mineral coating (unless theprecursor is solely a TEOS, in this case, a substantially mineralcoating would be produced, even in the case of curing temperatures lowerthan 400° C.).

Advantageously, a hollow cap, such as described previously, with abottom and a lateral wall rising from the bottom should be used as thesubstrate to be used to prepare the cooking item according toembodiments of the invention.

The substrate to be used in the frame of the present invention willadvantageously be made from a material selected from metals, glass, andceramics.

Advantageously, anodized or non anodized, aluminum or aluminum alloysubstrates, stainless steel, cast iron, iron or copper substrates may becited as the metal substrates which may be used in embodiments of themethod of the invention.

The method according to embodiments of the invention may furthercomprise a step of depositing an enamel layer on the side opposite tothat coated with a non stick coating according to the invention, thisenamel layer deposition step being carried out prior to the non stickcoating step according to embodiments of the invention.

Besides the aforementioned advantages, the implementation of the methodin accordance with embodiments of the invention is particularly simple,and this method may be considered without introducing importantmodifications to usual manufacturing processes of cooking item cookingitems.

Other advantages and features of the present invention will become moreapparent from the following description, given only by way of nonlimitative example and made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a cross-sectional schematic view of a cooking itemhaving a smooth internal side coated with a sol-gel type, vitreous,non-stick coating.

FIG. 2 represents a cross-sectional schematic view of a cooking itemhaving a sand blasted internal side coated with a sol-gel tpe vitreous,non-stick coating;

FIG. 3 represents a cross-sectional schematic view of a cooking item inaccordance with the invention according to a first alternativeembodiment (smooth substrate);

FIG. 4 represents a cross-sectional schematic view of a cooking item inaccordance with the invention according to a second alternativeembodiment (sand blasted substrate);

FIGS. 5 to 15 represent successive top views of a metal substrate coatedwith a non stick coating after impact resistance tests of the non stickcoating in accordance with the Erichsen test conforming to ISO standard6272 aimed to constitute a visual scale for the assessment of the impactresistance: FIGS. 5, 7, 9, 11, 13 and 15 are views of the substrateobtained when the impact is made on the internal side with the non stickcoating (internal drawing test), whereas FIGS. 6, 8, 10, 12 and 14 areviews of the substrate when the impact is made on the side opposite tothat provided with the non stick coating (exterior drawing test);

FIG. 16 represents a top view of the cooking item of FIG. 2 followingthe Erichsen test; and

FIG. 17 represents a top view of the cooking item of FIG. 4 followingthe Erichsen test.

DETAILED DESCRIPTION

The identical elements represented on FIGS. 1 to 4 are denoted byidentical numerical references.

FIGS. 1 to 4 represent, as a cooking item, a frying pan 1 comprising asubstrate 2 having a hollow cap shape with a bottom 24 and a lateralwall 25 rising from the bottom 34, as well as a grip handle 6. Thesubstrate 2 comprises an internal side 21 which may accommodate fooditems, and an exterior side 22 to be arranged on the heat source side,such as a cooking plate or a burner.

FIG. 1 particularly shows a cooking item whose smooth, substrateinternal side 21 is coated with a sol-gel type, vitreous, non stickcoating 4 in accordance with that of the present invention.

FIG. 2 shows a cooking item whose internal side 21 has been degreasedand then sand blasted and exhibits a surface roughness Ra between 3 μmand 8 μm. This sand blasted internal side 21 is also coated with asol-gel type vitreous, non stick coating 4 in accordance with thepresent invention.

FIGS. 3 and 4 show two alternative embodiments of a cooking item conformto the invention. With respect to these two alternative embodiments, theinternal side 21 is successively coated from substrate 2, with an enamelhard base layer 3 in accordance with the present invention and with avitreous, non stick coating 4, also in accordance with the presentinvention.

According to the alternative embodiment represented on FIG. 3, theinternal side 21 is smooth. In this case, the surface roughness Ra ofthe hard base layer 3 is comprised between 2 μm and 4 μm.

According to the alternative embodiment represented on FIG. 4, theinternal side 21 has been degreased and sand blasted beforehand so as toobtain a surface roughness Ra comprised between 3 and 8 μm. In thiscase, the surface roughness Ra of the hard base layer 3 is comprisedbetween 9 and 12 μm.

With respect to both alternative embodiments of the cooking itemaccording to the invention and represented on FIGS. 3 and 4, the hardlayer base 3 is a discontinuous enamel layer comprising a superficialdispersion of solidified enamel drops 31, having an average size between2 μm and 50 μm and homogeneously distributed on the surface of internalside 21, with a surface coverage rate of the internal side between 40and 80%, and a surface density comprised between 300 drops/mm² and 2000drops/mm².

With respect to both alternative embodiments, enamel drops 31 dispersedon the internal side 21 surface are bedded in the non stick coatinglayer 4, so as to cause the non stick coating 4 to stick to the enamelhard base layer 3. Such an enameled hard base layer 3 in the form of asuperficial dispersion of enamel drops makes it possible tosignificantly enhance the adherence of the non stick coating 4 over thesubstrate 2, and in particular, the impact resistance.

Hereafter, two embodiments of a cooking item 1 in accordance with theinvention according to the first or second alternative embodiment asrepresented on FIGS. 3 and 4 respectively. The methods for producing thecooking items include embodiments comprising the following successivesteps:

-   -   a) providing a substrate 2, exhibiting the final form of the        cooking item with an internal side 21 to be disposed on the side        of food items which would be introduced in said item 1, and an        exterior side to be disposed on a heat source side;    -   c) preparing a hard base layer 3 over the internal side 21 of        the substrate 2; then    -   d) preparing a non stick coating 4 on the hard base layer 3        formed in step c) through a sol-gel treatment according to        embodiments of the invention.

With respect to the first embodiment, step c) of preparing a hard baselayer 3 is directly carried out over the internal side 21 of thesubstrate, without this side being chemically or mechanically preparedbeforehand (smooth internal side 21).

Meanwhile, the second embodiment of the cooking item according to theinvention further comprises, between steps a) of providing the substrateand c) of preparing a hard base layer over the internal side of thesubstrate, a step b) of preparing the surface of the internal side 21comprising degreasing said surface followed by a mechanical treatingsuch as sand blasting or brushing.

With respect to both such embodiments of the invention, step c) ofpreparing the hard base layer 3 comprises the following successive stepsof:

c1) preparing an aqueous slurry of enamel frit, said enamel frit havingless than 50 ppm of cadmium, and comprising 30 to 40% by weight ofsilica and 15 to 30% by weight of titanium oxide, less than 10% byweight of vanadium oxide and less than 4% by weight of lithium oxidewith respect to the total weight of the frit, the aqueous slurrycomprising less than 20% by weight of mineral fillers with respect tothe total weight of the slurry,

c2) applying, through air spraying, the slurry over the internal side 21of substrate 2 with a spraying pressure higher than or equal to 4 barsand an amount of enamel deposition over said internal side 21 comprisedbetween 0.07 g/dm² and 0.2 g/dm² (in this case, an amount of enamelslurry of 0.8 for a frying pan whereof the bottom has a diameter of 26cm); then drying;

c3) drying (at a temperature between 120° C. and 200° C.) and/or curingsaid enamel layer 3 (at a temperature between 540 and 580° C. for atleast 5 to 7 minutes, preferably, of about 550° C.), the drying and/orcuring being carried out before the preparation of the non stick coating4 of said enamel layer 3.

Advantageously, the enamel frit comprises:

Al₂O₃: less than 1%;

B₂O₃: less than 1%;

BaO: less than 1%;

K₂O: from 5 to 20%;

Li₂O: less than 4%;

Na₂O: from 10 to 25%;

P₂O₅: less than 4%;

SiO₂: from 30 to 40%;

TiO₂: from 15 to 30%;

V₂O₅: less than 10%;

The indicated contents are weight percents with respect to the totalweight of the frit.

Advantageously, the enamel frit slurry further comprises:

quartz: from 5 to 30%;

SiC: from 10 to 30%;

Pigment: from 1 to 10%;

Suspension agent: from 2 to 10%;

The indicated contents are weight percentages with respect to the totalweight of the slurry.

The following examples illustrate the invention without limiting itsscope.

In the examples, unless otherwise specified, all the amounts are givenin grams.

EXAMPLES Products

Enamel Frit:

Al₂O₃

B₂O₃

B₄O

K₂O

Li₂O

Na₂O

P₂O₅

SiO₂

TiO₂

V₂O₅

Sol-Gel Coating (A+B):

Aqueous Composition A:

Colloidal metal oxide: colloidal silica in the form of an aqueoussolution with 30% of silica, from Clariant under trade name Klebosol.

Solvent: isopropanol

Silicone oil: food-grade methylsilicone oil from TEGO under trade name“TEGO ZV 9207”.

Pigment: mineral black pigment from Ferro under trade name “FA 1260”.

Solution B:

Precursors:

-   -   methyltriethoxylsilane (MTES) of formula Si(OCH₂CH₃)₃CH₃,    -   methyltriethoxylsilane (MTMS) of formula Si(OCH₃)₃CH₃,

Acid: acetic acid

Test

Impact Resistance of the Non Stick Coating Assessed by Means of theErichsen Test in Accordance with ISO Standard 6272.

The test is an impact testing through which a 2-kg ball is dropped froma height of 50 cm. For the sake of testing, aluminum pads, of which oneside is coated with a sol-gel type non-stick coating in accordance withthe invention, are used. All the pads are identical to each other (interms of thickness and alloy nature) so as to obtain a constant strainfor all the tests.

This test comprises performing an impact directly on the non-stickcoating deposited on the coated side of a pad (interior drawing test)and an impact on the side opposite to that coated with a non stickcoating of another pad (exterior drawing test).

After the impacts have been performed, a visual examination of thecoated side of the non stick coating is carried out.

The impact resistance of the non stick coating is assessed on the basison the following visual scale, which is established after a directimpact on the non stick coating, on one hand (internal drawing test),and on the side opposite to the coated side of the non stick coating(exterior drawing test) on the other hand:

-   -   rank 0 is given if the following observation is made:    -   with regard to the interior drawing test:    -   at the impact, a full delamination of the non stick coating over        the entire deformed surface of the internal side which is        expressed by the appearance of a “white” area at the impact        (corresponding to a coating-free metal surface portion), shaped        as a disk whose diameter is of about 25 mm, such as illustrated        in FIG. 5;    -   with regard to the exterior drawing test:    -   also, a complete delamination of the non stick coating over a        large surface of the deformed internal side, which is expressed        by the appearance of a “white” area at the impact (corresponding        to a coating-free metal surface portion), this area being shaped        as a disk of at least 10 mm in diameter, such as illustrated in        FIG. 6;    -   rank 1 is given if the following is observed:    -   with regard to the interior drawing test:    -   at the impact, an almost complete delamination of the non stick        coating over a large surface of the deformed internal side, this        deformation being expressed by the appearance of a quasi “white”        area at the impact (corresponding to a coating-free metal        surface portion), this area being shaped as a disk whose        diameter is of about 20 mm as illustrated on FIG. 7;    -   with regard to the exterior drawing test:    -   a quasi-complete delamination of the non stick coating over an        average surface of the deformed internal side, which is        expressed by the appearance of a “white” area with no coating at        the impact, shaped as a disk having a diameter of about 10 mm,        such as illustrated on FIG. 8;    -   rank 2 is given is the following is observed:    -   with regard to the interior drawing test:    -   at the impact, an almost complete delamination of the non stick        coating over an average surface of the coated internal side,        which is expressed by the appearance of a “white”, coating-free,        area at the impact, being shaped as a disk whose diameter is of        about 10 mm as illustrated on FIG. 9;    -   with regard to the exterior drawing test:    -   at the impact, a white area shaped as a disk having a diameter        less than 10 mm, wherein large slivers reaching the substrate        metal surface are located, with a relatively high sliver        density, such as illustrated on FIG. 10;    -   rank 3 is given if the following is observed:    -   with regard to the interior drawing test:    -   at the impact, a central area in which are located large slivers        exposing the metal, such as illustrated on FIG. 11;    -   with regard to the exterior drawing test:    -   at the impact, a white area shaped as a disk having a diameter        less than 10 mm, and in which small slivers reaching the metal        are located, with a fairly high density, such as illustrated on        FIG. 12;    -   rank 4 is given if the following is observed:    -   with regard to the interior drawing test:    -   at the impact, an area shaped as a disk having a small diameter,        wherein fine slivers reaching the metal are located, with a        rather low density, such as illustrated on FIG. 13;    -   with regard to the exterior drawing test:    -   several pin holes reaching the metal, located at the impact,        such as illustrated on FIG. 14;    -   and finally, rank 5 is given if the following is observed:    -   with regard to the interior drawing test:    -   at the impact, a small halation, lighter than the non stick        coating, such as illustrated on FIG. 15;    -   with regard to the exterior drawing test:    -   no modifications of the aspect of the non stick coating (non        illustrated in the visual scale used in the present        application).

Example 1 Preparation of an Enamel Frit F in Accordance with the EnamelFrit Used in a Method in Accordance with an Embodiment of the Invention

An enamel frit F in accordance with the enamel frit used in a method inaccordance with an embodiment of the invention has been prepared bymelting the following components:

Al₂O₃: 0.1%;

B₂O₃: 0.6%;

BaO: 0.3%;

K₂O: 12%;

Li₂O: 2.3%;

Na₂O: 19%;

P₂O₅: 1.6%;

SiO₂: 35%;

TiO₂: 23.5%;

V₂O₅: 5.2%;

Then, the melted mixture obtained is grinded to yield a powdery frit Fwhose mean particle size is of 15 μm and a linear expansion coefficientof 494×10⁻⁷ m·K⁻¹.

Example 2 Preparation of an Example of Enamel Frit Slurry B′, inAccordance with the Enamel Frit Used in Method in Accordance with anEmbodiment of the Invention

The enamel frit F has the form of a slurry B, by mixing the followingcomponents (parts by weight):

Enamel frit F: 70;

Water: 55;

Quartz: 25;

SiC: 23;

Fe and Mn oxide based black pigment: 5.

Boric acid: 4

Thus obtained slurry B′ exhibits a density of 1.70 g/cm³ and a “set-up”of 1300 g/m².

Here, the meaning of “set-up” is the amount of matter required touniformly cover a given surface after application.

Example 3 Preparing a Composition of Non Stick Coating R in Accordancewith an Embodiment of the Present Invention

In view of preparing a composition of non stick coating R in accordancewith the invention, the following is carried out:

3.1) preparing a colloidal silica based aqueous composition inaccordance with a method pursuant to an embodiment of the invention;

3.2) and 3.3) preparing a MTES or MTMS based solution B, in accordancewith the embodiment;

3.4) and 3.5) performing, from aqueous solution A and solution B, asol-gel composition (A+B), in accordance with a method pursuant to anembodiment of the invention.

3.1 Preparation of a Colloidal Silica Based Aqueous Composition A

A colloidal silica based aqueous composition A, presented in table 1,has been performed.

TABLE 1 Components of portion A Amount (g) Aqueous solution with 30%colloidal silica: 42 Klebosol Water 16 Isopropanol 8 Black pigment FA1220 33 Silicone oil TEGO ZV 9207 1 Total 100

3.2 Preparation of an MTES Based Solution B

A solution B is prepared by mixing 59.4 g of MTES with 2.4 g of aceticacid, yielding a solution of 4% by weight of acid in the MTES.

3.3 Preparation of an MTMS Based Solution B According to an Embodimentof the Invention

A solution B is prepared by mixing 59.4 g of MTMS with 0.6 g of aceticacid, yielding a solution of 1% by weight of acid in the MTMS.

3.4 Preparation of a First Example of Sol-Gel (SG) Composition Accordingto an Embodiment the Invention (from MTES)

A first sol-gel SG composition according to an embodiment of theinvention is made by adding, to 100 g of aqueous solution A according toan embodiment of the invention, 61.8 g of solution B of example 1.2.Mixing in a planetary mixer for one hour at a temperature lower than 60°C. yields a sol-gel composition according to an embodiment of theinvention SG which is kept at room temperature. The SG composition isleft to mature for 24 hours at room temperature after mixing, beforeapplication over a substrate.

3.5 Preparation of a Second Example of Sol-Gel (SG) CompositionAccording to an Embodiment of the Invention (from MTMS)

A first sol-gel SG composition according to an embodiment of theinvention is made by adding, to 100 g of aqueous solution A according toan embodiment of the invention, 60 g of solution B of example 1.3.Mixing in a planetary mixer for one hour at a temperature lower than 60°C. yields a sol-gel composition according to an embodiment of theinvention SG which is kept at room temperature. The SG composition isleft to mature for 24 hours at room temperature after mixing, beforeapplication over a substrate.

Example 4 Preparing a First Example of a Cooking Item According to anEmbodiment of the Invention Sand Blasted Substrate+DiscontinuousEnameled Hard Base Layer

As the substrate, an aluminum cap obtained by forming an aluminum disc(1200 type), the accordingly obtained cap having a bottom whose diameteris of about 26 cm.

This cap is degreased by spraying an alkaline solution, and the bottominternal side of the cap is sand blasted (using corundum) yielding asurface roughness between 2 and 4 μm.

Then, on the sand blasted internal side, slurry B′ is applied b means ofan air gun, so as to form a discontinuous layer shaped as disconnecteddroplets. The cap accordingly coated is dried at a temperature of 140°C., then vitrified at 555° C. for 5 minutes, so as to obtain adiscontinuous hard which thickness is of 50 μm. Roughness Ra of thiscontinuous hard base layer is comprised between 9 μm and 12 μm.

Following cooling of this hard base layer, non stick coating R iscarried out by applying the sol-gel composition SG of example 3.4(obtained from a MTES based solution B) over the discontinuous enameledhard base layer 3, according to the following cycle:

-   -   applying a sol-gel composition layer SG of example 3 over the        substrate, to a wet thickness of 40 to 70 microns,    -   drying for 1 minute at 60° C., and    -   cooling at room temperature.

It is possible to apply this cycle several times, the number of cyclesbeing determined by the desired final thickness.

At the end of the application/drying cycle or cycles, curing is carriedout for 15 minutes at 270° C. Thus, a cooking item having a non stickcoating exhibiting a thickness comprised between about 35 microns (±5μm) and which is smooth, black and glossy is obtained.

The cooking item of example 4 according to an embodiment of theinvention is shown on FIG. 4.

Example 5 Preparation of a Second Example of a Cooking Item According toan Embodiment of the Invention Sand Blasted Substrate+DiscontinuousEnameled Hard Base Layer

The same as for example 4 is carried out except that sol-gel compositionSG of example 3.5 is used (obtained from a MTMS based solution B).

Cooking item of example 5 is also shown on FIG. 4.

Example 6 Preparing a Third Example of a Cooking Item According to anEmbodiment of the Invention Smooth Substrate+Discontinuous Enameled HardBase Layer

As the substrate, the same substrate as example 4 is used except thatinternal side 21 of substrate 2 is not subjected to a mechanicaltreatment and remains smooth.

Meanwhile, with regard to the preparation of the enamel layer 3, thesame as for example 4 is carried out.

With regard to the preparation of the sol-gel coating SG over hard baselayer 3, the same as for example 4 is carried out, using sol-gelcomposition SG of example 3.4 (obtained from an MTMS based solution B).

Cooking item of example 6 is shown on FIG. 3.

Example 7 Preparation of a Second Example of a Cooking Item According toan Embodiment of the Invention Sand Blasted Substrate+DiscontinuousEnameled Hard Base Layer

The same as for example 6 is carried out except that sol-gel compositionSG of example 3.5 is used (obtained from a MTMS based solution B).

Cooking item of example 7 is also shown on FIG. 3.

Example 8 Preparation of a First Example of a Reference Cooking ItemSand Blasted Substrate but No Hard Base Layer

As the substrate 2, the same substrate as for example 4 is used (sandblasted substrate).

In this example, no discontinuous enameled hard base layer is performedover internal side 21 of substrate 2. Consequently, the non stickcoating 4 is directly made over the substrate internal side 21, sandblasted then degreased beforehand, proceeding in the same manner as forexample 4, that is, using the MTES based sol gel coating of example 3.4.

The cooking item of example 8 is shown on FIG. 2.

Example 9 Preparation of a Second Example of a Reference Cooking ItemSand Blasted Substrate but No Hard Base Layer

The same as for example 8 is carried out except that sol-gel compositionSG of example 3.5 is used (obtained from a MTMS based solution B).

Cooking item of example 9 is shown on FIG. 2.

Example 10 Preparation of a Third Example of a Reference Cooking ItemSmooth Substrate but No Hard Base Layer

As the substrate 2, the same substrate as for example 6 is used (smoothsubstrate). Then, the non stick coating 4 is made directly over theinternal side 21 of substrate 2 which remained smooth, in a manneridentical to example 8 (MTES based sol-gel coating of example 3.4). 7

Cooking item of example 10 is represented on FIG. 1.

Example 11 Preparation of a Third Example of a Reference Cooking ItemSmooth Substrate but No Hard Base Layer

The same as for example 10 is carried out except that sol-gelcomposition SG of example 3.5 is used (obtained from a MTMS basedsolution B).

Cooking item of example 11 is shown on FIG. 1.

Example 12 Erichsen Test

The impact resistance is assessed according to the Erichsen test bysubjecting reference cooking items of examples 8 to 11 (shown on FIGS. 2and 1 respectively) and cooking items of examples 4 to 7 (respectivelyshown on FIGS. 4 and 3) according to embodiments of the invention.

The Erichsen tests results obtained from frying pans, according to theinvention, from examples 4 and 5 (sand blasted substrate, FIG. 4), and 6and 7 (smooth substrate, FIG. 3), on the one hand, and reference fryingpans of examples 8 and 9 (sand blasted substrate, FIG. 2), and 10 and 11(smooth substrate, FIG. 1) on the other hand, are visual observationstranslated into a ranking varying from 0 to 5 according to the visualscale indicated above and illustrated by FIGS. 5 to 15, which are all ofthe same scale. These results are presented in table 2 below.

TABLE 2 Erichsen Test results: ranking according to the visual scale(1-5) Interior Exterior drawing drawing Cooking item according to anembodiment of 3 4 the invention with sand blasted substrate (MTES, ex.4, FIG. 4) Cooking item according to an embodiment of 3 4 the inventionwith sand blasted substrate (MTMS, ex. 5, FIG. 4) Cooking item accordingto an embodiment of 1 4 the invention with smooth substrate (MTES, ex.6, FIG. 3) Cooking item according to an embodiment of 1 4 the inventionwith smooth substrate (MTES, ex. 7, FIG. 3) Cooking item according to anembodiment of 2 2 the invention with sand blasted substrate (MTES, ex.8, FIG. 2 and 16) Cooking item according to an embodiment of 2 2 theinvention with sand blasted substrate (MTMS, ex. 9, FIG. 2 and 16)Cooking item according to an embodiment of 0 0 the invention with smoothsubstrate (MTES, ex. 10, FIG. 1) Cooking item according to an embodimentof 0 0 the invention with smooth substrate (MTMS, ex. 11, FIG. 1)

FIG. 16 represents (on the same figure) the state of the non stickcoating after subjecting a sand blasted metal substrate comprising a nonstick coating 4 with no hard base layer 3 to an internal drawing test(left part on FIG. 16) on the one hand, and after an exterior drawingtest (right part of FIG. 16) on the other hand.

FIG. 16 corresponds to FIGS. 9 and 10 of visual scale (ranking 2).

FIG. 17 represents (on the same figure) the state of the non stickcoating after subjecting a sand blasted metal substrate successivelycovered with a hard base layer 3 then a non stick coating 4 inaccordance with an embodiment of the present invention, to an internaldrawing test (left part on FIG. 17) on one hand, and after an exteriordrawing test (right part of FIG. 16) on the other hand.

The results presented in table 2 and illustrated on FIGS. 16 and 17 showthat the formation of an enameled hard base layer over the internal sidesurface makes it possible to improve the adhesion properties of asol-gel type non stick coating to a substrate.

Prior sand blasting further reinforces the adhesion properties to thesubstrate. Nevertheless, only sand blasting the substrate (without ahard base layer in accordance with the invention) does not providesatisfactory results in terms of adhesion to the substrate.

The invention claimed is:
 1. A method for producing a cooking item, the method comprising: a) providing a substrate having an end shape of the cooking item, the end shape presenting a concave internal side configured to contain a food item introduced into said cooking item, and a convex exterior side configured to be positioned proximate a heat source; b) optionally, treating the internal side of the substrate, so as to obtain a treated internal side adapted to cause a hard base layer to adhere to the substrate; c) preparing a discontinuous enameled adhering base layer on said internal side of the substrate, be it treated beforehand or not; and d) preparing a non stick PTFE-free coating on said hard base layer formed in step c); wherein step c) of preparing a discontinuous, enameled hard base layer comprises successively— c1) preparing an aqueous slurry of enamel frit, c2) applying, through air spraying, the slurry on the internal side of the substrate with a pressure equal to or higher than 4 bars such that the amount of deposited slurry is comprised between 0.07 g/dm² and 0.2 g/dm², and then c3) drying and/or curing said enamel layer so as to form a hard biscuit of enamel or a layer of cured enamel, and wherein step d) of preparing the non stick PTFE-free coating on said enamel hard biscuit or said cured enamel layer comprises successively— d1) preparing a sol-gel composition (A+B) comprising at least a colloidal metal oxide and at least a metal oxide precursor, wherein the preparation of the sol-gel composition (A+B) comprises successively: i) preparing an aqueous composition A comprising 5 to 30% by weight with respect to a total weight of the aqueous composition A of at least one colloidal metal oxide, and 0 to 20% by weight with respect to a total weight of composition A of a solvent comprising at least one alcohol; ii) preparing a solution B comprising at least a metal alkoxide as the at least one metal oxide precursor; iii) mixing solution B of metal alkoxide with aqueous solution A so as to obtain a sol-gel composition (A+B) containing 40 to 75% by weight of the aqueous composition A with respect to a total weight of the sol-gel composition (A+B), so that an amount of colloidal metal oxide represents 5 to 30% by weight of the sol-gel composition (A+B) on a dry basis; d2) applying over an entirety or over a portion of the enamel hard biscuit or said cured enamel layer at least a layer of the sol-gel composition (A+B) having a wet-basis thickness of at least 20 μm, and then d3) curing said sol-gel composition layer (A+B) so as to obtain a non stick PTFE-free vitreous coating having a thickness of at least 10 μm.
 2. The method according to claim 1, wherein: said enamel frit comprises 30 to 40% by weight of silica and 15 to 30% by weight of titanium oxide, less than 10% by weight of vanadium oxide and less than 4% by weight of lithium oxide with respect to a total weight of the enamel frit, and the aqueous slurry comprises at least 20% by weight of mineral fillers with respect to a total weight of the aqueous slurry.
 3. The method according to claim 1, wherein the colloidal metal oxide is selected from the group consisting of silica, alumina, cerium oxide, zinc oxide, vanadium oxide, zirconium oxide, and combinations thereof.
 4. The method according to claim 1, wherein the aqueous composition A further comprises at least one silicone oil.
 5. The method according to claim 1, wherein the metal alkoxide of solution B is selected from the group consisting of: precursors having a chemical formula of M₁(OR₁)_(n), precursors having a chemical formula of M₂(OR₂)_((n-1))R₂′, precursors having a chemical formula of M₃(OR₃)_((n-2))R₃′₂, with: R₁, R₂, R₃ or R₃′ being an alkyl group, R₂′ designating an alkyl or phenyl group, n being an integer corresponding to a maximal valence of metals M₁, M₂, or M₃, M₁, M₂, or M₃ designating a metal selected from among Si, Zr, Ti, Sn, Al, Ce, V, Nb, Hf, Mg or Ln.
 6. The method according to claim 5, wherein the metal alkoxide is an alkoxysilane selected from methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS).
 7. The method according to claim 1, wherein step b) of treating the internal side of substrate comprises a step of performing a sand blasting type mechanical treatment.
 8. The method according to claim 1, wherein during step c) of preparing a discontinuous, enameled hard base layer, the non cured enamel layer is dried at a temperature between 120 and 200° C., so as to obtain an enameled biscuit.
 9. The method according to claim 1, wherein during step c) of preparing a discontinuous, enameled hard base layer, the non cured enamel layer is subjected to a curing at a temperature between 500° C. and 580° C. 